With the recent trend of portable electronic devices toward miniaturization and high performance, there is an increasing demand for secondary batteries used as the power source for such electronic devices. In particular, non-aqueous electrolyte secondary batteries such as lithium ion secondary batteries, which have high energy density and can be easily made more light-weight, are being actively developed. As electronic devices are increasingly becoming higher in performance and functionality and more compact, non-aqueous electrolyte secondary batteries are required to provide higher capacity.
In non-aqueous electrolyte secondary batteries, electrodes (positive and negative electrodes) are usually produced, for example, by applying a paint containing an active material, a binder, a conductive agent, etc. onto a surface of a current collector made of a long strip-like metal foil to form an active material layer. The current collector is connected to an external terminal, such as a battery case or seal plate, with an electrode lead made of a rectangular metal strip.
The current collector is connected to the electrode lead at the part where the current collector is exposed at the surface of the electrode, i.e., the exposed part of the current collector, since if the connection is made through the active material layer, sufficient electrical continuity cannot be obtained. The exposed part of the current collector is formed by removing the active material layer on a part of the current collector over the whole width, or not applying the paint onto a part of the current collector over the whole width (see PTLs 1 and 2).
Also, it has been proposed to connect a current collector and an electrode lead, for example, by placing the electrode lead on the exposed part of the current collector, applying a fluing process to the overlapping part from the electrode lead side so as to penetrate through the electrode lead and the current collector and cause a part of the electrode lead to penetrate through the current collector, and crimping the part of the electrode lead (see PTLs 3 and 4).